The present invention relates to a process for preparing epoxy-modified polyamide resins, the epoxy-modified polyamide resins obtained from the process, and the epoxy-modified polyamide resin films.
Polyamide resins have high polarity and strong hydrogen bond with a high bond energy between the nitrogen and carbon atoms in the amide linkage, so that they are excellent in heat resistance and widely used for applications where heat resistance is required.
As means for the preparation of such polyamide resins, a method is known in which a diisocyanate and a dicarboxylic acid and/or a tricarboxylic anhydride are subjected to carbon dioxide gas elimination polymerization in a nitrogen-containing aprotic solvent. The produced polymerization solution can be put to use as a paint, adhesive or such in the form as it is. According to this method, however, it is difficult to obtain polyamide resins of high molecular weight, and the obtained polyamide resins are not always satisfactory in their film properties, especially flexibility.
In Japanese Patent Unexamined Publication (JP-A) Nos. 5-51447, 5-230169, 5-51571 and 5-230430, there is disclosed a process for preparing an epoxy-modified polyamide resin by removing a nitrogen-containing aprotic solvent from a polyamide resin solution prepared by a known method, purifying the polyamide resin, dissolving it in a nitrogen-free aprotic solvent again, followed by reacting with an epoxy resin.
Further, in JP-A No. 2-22319, there is disclosed a process for preparing an epoxy-modified polyamide resin without using a solvent.
However, the obtained epoxy-modified polyamide resins are not always satisfactory in their heat resistance.
Further, since the process described in the JP-A Nos. 5-51447, 5-230169, 5-51571 and 5-230430 includes the step of removing a nitrogen-containing aprotic solvent from a polyamide resin solution and the step of dissolving it in a nitrogen-free aprotic solvent again, this process is placed at a disadvantage in respect of the cost for manufacturing the epoxy-modified polyamide resin.